stefan.ballmer@LIGO.ORG - posted 23:44, Tuesday 12 May 2015 - last comment - 11:48, Wednesday 13 May 2015(18397)
SRCL FF coupling remeasured
Sheila, Evan, Stefan We remeasured the required SRCL FF filter today. The transfer function changed significantly. The only thing we know we changed was the POP_45 phase (H1:LSC-POP_A_RF45_PHASE_R) from 66deg to 58deg - this minimized a SRCL drive in MICH. The fit functions are roughly: May 11: p:0,0 z:2.28571+31.9183i,2.28571-31.9183i,-180 (M1) May 12: p:0,0 z:2.28571+47i,2.28571-47i,-600 (M2) The 1st attached plot shows these two measurements, overplayed with the models M1 and M2. The 2nd plot shows the 4 raw measurements (2 yesterday, 2 today). We didn't try to engage this SRCL FF yet.
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At the end of the day - after sitting at 10W for a while, and with the MICH FF optimized - we measured the SRCL FF transfer function again. It looked a lot closer to yesterday's measurement.
Those changes in the SRCL to DARM couplings are consistent with the simulation results for a changing SRCL offset. The reason of this changing offset can be related to many things, for example alignment.
Looking at the data and the simulation results, to have a zero at 30-50 Hz we need offsets of the order of 6-8 nm